Single Storm in Mexico Kills 75%
of Eastern North America?s Migratory Monarchs
January 2002

Between 12-16 January 2002, a severe winter storm hit the monarch sanctuary region deep in central Mexico. Mexico's over-wintering sites harbor all of eastern North America's migratory monarch breeding stock. Dr. Lincoln Brower and colleagues released mortality estimates. Based on data collected from the two largest sanctuaries, over 75% of the population was killed by this single storm.

  • Sierra Chincua Sanctuary: 74% of the butterflies were killed
  • El Rosario Sanctuary: 80% of the population was killed
  • At the two colonies combined, an estimated 200-272 million butterflies were killed.

Significantly, these two huge colonies are the winter sanctuaries of 2/3 of eastern North America?s migratory butterflies. The other 1/3 of the butterflies are spread among other smaller sites in the vicinity. While scientists have not yet visited these outlying sites, mortality rates are feared to be similar because the sites are small, their forest habitat is less pristine, and because the rain and cold were prolonged in the region.

All Photos by Dave Kust

Dr. Brower described the catastrophe in an interview with Journey North: ?The ground in these two colonies was littered with monarchs that had an eerie flat, pallid appearance that I have never before seen--like wet leaves. The heavily packed piles of butterflies were up to 13 inches deep, and even those that were still alive may not have been able to struggle out. The main survivors were buried alive, covered by dead butterflies that were insulating them (from the cold).?

In an interview with National Public Radio Brower said, ?The most macabre aspect of the whole thing was sticking my hands down in and realizing at the bottom of the pile there were a whole bunch of them still alive.?

Killing Conditions: Wet Followed by Extreme Cold
For monarchs, the weather pattern that occurred in mid-January was the recipe for disaster: Heavy rains were followed by clearing skies and plummeting temperatures. This storm was an historic extreme in that the rains were heavy during the peak of the dry season in Mexico, and the temperatures were exceptionally cold. Monarchs are essentially tropical butterflies and cannot tolerate sub-freezing temperatures for very long. When they are wet, they die at warmer temperatures than they would if dry.

Said Brower, ?Never in the 27 years we?ve been studying in Mexico have we known temperatures to have fallen this low. I think the temperatures dropped below -8C and froze all the wet butterflies...except those in the middle of the clusters...?

It?s important to emphasize that this year?s storm was an extreme, and a high level of mortality may have occurred even with the best forest cover. However, for scientists, conservationists and government officials alike, the historic storm of 2002 will serve as a cautionary reminder: If deforestation continues at its present rate, a single storm even less severe would be capable of decimating the over-wintering population.

?There's always a probability of a severe storm like this,? Brower told NPR, ?but at the hands of humans there's been a 44 percent degradation of the forest over the past 28 years. And the rate of forestry in the area is increasing. This storm shows that we need to really protect the central core area where their survival is really threatened.?

Meteorologist?s Perspective: Satellite Views of the Storm
We wondered whether satellite images were available to document the strength and scope of the storm and cold front. Dr. Dave Dempsey of San Francisco State University responded with these impressive animated video loops showing the jet stream, visible and infrared views. (The sanctuary region is marked with a red "+".) From Dr. Dempsey:

"A cold front swept across central Mexico from the northwest, followed by cold, clear weather. The infrared and visible satellite loops (below left and center) show the broad band of clouds that accompanied the cold front and produced rain and even snow. A cold front is the leading edge of a "tongue" of cold air that protrudes southward from its source farther north. High in the atmosphere, the jet stream blows around the edges of the tongue, forming a dip or "trough" in the jet stream pattern. The jet stream loop (below right) shows a narrow trough dropping unusually far south into central Mexico, indicating the arrival of a tongue of cold air." From here, the most useful addition would be a quantitative estimate of the relative rarity of the January event which requires a familiarity with the climatological data base that I don't have (though I have no doubt that someone out there does)."

Click to View Full-sized Images
Courtesy of Dr. Dave Dempsey, San Francisco State University

Natural Selection: Meet the Survivors
From an evolutionary perspective, scientists are interested in this event as an example of natural selection at work. Just think: The monarchs that migrate across Eastern North America in the future--perhaps to your own backyard--will be the offspring of those that survived the storm. Any characteristic that helped them to survive will be in the gene pool of future monarchs. For example, might these survivors:
  • Be more tolerant to cold than the ones that died?
  • Be stronger, fatter, or otherwise more physically fit than those who died?
  • Can you think of other possible differences, physical or behavioral, that enabled some to survive?

...or was it just chance!?

Outlook for the Future
Scientists are now left wondering how long it will take for the monarch population to rebound. Mexico's over-wintering sites harbor all of eastern North America's migratory monarch breeding stock. What long-term effect will this storm have on monarch populations, given such a high mortality rate?

First, it?s important to remember that monarchs, as insects, are capable of high reproduction rates. A single female can lay hundreds of eggs. Second, scientists simply don?t know enough about monarch population dynamics to make predictions with confidence. Dr. Orley Taylor, Director of Monarch Watch, wrote an excellent analysis of the storm (see link below) and made this important observation:

"This is the lowest known number of butterflies at these sites over the last 27 years. In retrospect, it appears to be fortunate that the January freeze occurred this year, a year in which the overwintering population was robust (about 100 million butterflies). Even though estimated mortality due to the January freeze is extremely high (>80%) the number of surviving butterflies may be sufficient to recolonize the breeding areas without a long-term depression of the population. Had this storm occurred last season when the overwintering population was at an all time low (28.3 million), it is likely that it would have taken the population many years to return to normal levels of 60-120 million overwintering butterflies.?

Storm in Monarch Sanctuaries: Links to Additional Information

Spring Migration Data Takes on New Importance
In the aftermath of this event, scientists will watch with interest as the remaining butterflies move north this spring to breed.

?It will be important to follow the population closely this spring as it moves north," said Dr. Karen Oberhauser, who encourages volunteers to participate in Journey North's migration study and her own study of monarch reproduction, the Monarch Larva Monitoring Project. "...this will give us realistic estimates of the cascading effects of the storm mortality.?

Journey North: Spring Monarch Migration Study
Observers report the first sightings of adult monarchs, the emergence of first milkweed (the host-plant monarchs need to reproduce), and the first evidence of reproduction (first eggs, first larva).

Monarch Larva Monitoring Project
The overarching goal of the project is to better understand how and why monarch populations vary in time and space, with a focus on monarch distribution and abundance during the breeding season in North America. On a weekly basis throughout the breeding season, observers monitor monarch reproduction at local sites.

Estimating Monarch Mortality: The Scientists? Methods
And this exactly what the team did. As these pictures show, Dr. Brower, Dave Kust, his family and visiting friends, collected hundreds and hundreds of butterflies from the sanctuary floor. They selected butterflies by making quadrants measuring .25 meters square, chosen randomly in the colony. They gathered all of the butterflies from inside each measured area. These are called ?measured area samples.?







Twenty nine random samples were taken from the Rosario and Chincua colonies. The team then carried the butterflies down to a lower a elevation where it?s warmer--the Kust?s house in downtown of Angangueo. For several days they worked through the samples, separating the butterflies as alive, dead, or ?moribund.? (Moribund means the butterfly is not yet dead, but is flight impaired even when warm, and may die in the future.)







Here are the data from the six samples from Chincua. Together they represent a combined area of 1.5 square meters. They counted 4,288 butterflies of which:

  • 599 were still alive (13.9%).
  • 327 were moribund (7.6%)
  • and the rest, 3362 were dead (78%).

Thus the density of dead butterflies was 3,362 per 1.5 meter squared. This works out to be 2,241 dead per meter squared, or 22.41 million dead monarchs per hectare.

Population Estimates Before and After the Storm

Post-storm population estimate assumes 80% morality at all colonies, based on preliminary data.

Warming the butterflies would resolve the question about WHETHER the grounded butterflies were truly dead, and counting them revealed the percent killed. But what about the living butterflies? How many remained? To estimate the damage they realized they needed to know:
  • The population size BEFORE THE STORM
  • The population size AFTER THE STORM

Fortunately, every winter Eligio Garcia, of Mexico?s Instituto Nacional de Ecologia, measures the colonies, and had done so before the storm. The size of the colony is estimated in two different ways: 1) by measuring the area the colony occupies, and 2) by counting the number of trees filled with butterflies. After the storm the Chincua and El Rosario were revisited. (Other sites are still to be measured.) Here are the results:
Sanctuary BEFORE Storm AFTER Storm

Percent Killed

Chincua 2.96 hectares 0.76 hectares


El Rosario 2.69 hectares Not Measured*


El Rosario 942 trees filled 187 trees filled


* In El Rosario it was not possible to measure the area occupied by the butterflies after the storm, due to dispersal of the colony into loosely aggregated clusters. For this reason, tree counts were used.

The annual mid-winter population estimates, shown in the graph above, were made by Eligio Garcia, of Mexico?s Instituto Nacional de Ecolocia. We estimated the post-storm population size by assuming 20% of the population survived. Much more information is needed to validate this estimate; it?s based on observations at only two colonies, for example, and the mortality rate might be much higher at the smaller, outlying colonies.

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